Thermocouple



Aug- 13, l957 G. SCHNEIDER ETAL THERMOCOUPLE Filed April l250, 1956 l ll l l l l l ATTORNES United States Patent iOtlice Patented` Aug. 13,1957 l 2,802,894 THERMOCOUPLE Gnther Schneider, Frankfurt am MainEschersheim, and YAlfred Boettcher, Hanau am Main Hohe Tanne, Germany,assignors to Deutsche Goldund Silber-Scheideanstalt vormals Roessler,Frankfurt am Main, Germany Application April 30, 1956, Serial No.58i,479 .Claims priority, application Germany April 30, 1955 9 Claims.(Cl. 13G-S) The invention relates to apparatus for measurement of hightemperatures and especially between 1500 and 2000 C. with the aid ofthermocouples formed of high melting metals and their alloys.

It is known to employ thermoelements in the form of the so-calledthermocouples for the measurement of temperatures. The best knownthermocouple elements are those of nickel/ chromium-nickel andplatinum/platinum-rhodium.

The simplicity of the use of thermocouples, as well as their accuracy,renders it desirable to provide thermocouples for use at temperaturesabove l500 C. However, up until now, the previous attempts employingcombinations of the known high melting metals have led only to limitedsolutions. The thermocouples produced either did not meet thetechnological requirements or were only of limited application. Thereason for this was that the wire employed for the thermocouples eitherstill had too low a melting point or the sensitivity thereof especiallyagainst thermal influences, which can easilyA lead to undesirableembrittlement and therefore early destruction of the elements or atleast to a diminished accuracy. In a similar manner, a change in alloycomposition through evaporation of one of the alloy components duringuse of thermocouples at high temperatures can lead to diiiiculties.Also, in platinum metals, the sensitivity to the so-called platinumpoisons is disadvantageous. Furthermore, it is, of course, alsonecessary to select such combinations for thermocouples that asuiiciently high thermoelectric E. M. F. is provided. For example,thermocouples of Ta/W, Mo/Ta and Mo/W only provide a relatively lowthermoelectric E. M. F. and also the change in thermoelectric E. M. F.with change in temperature is too low for practical purposes.

According to the invention, it was found that excellent thermocouplescould be produced for the measurement of high temperatures fromcombinations of iridium paired with rhenium-iridium alloys or ofiridium-rhenium alloys paired with rhenium-iridium alloys of differentcomposition. Preferably, the thermocouples according to the inventionare sealed in suitable gas tight ceramic armatures in a known manner toprotect them fully from the influence of the atmosphere or at least tosuch an extent that the life of the thermocouples is not deleteriouslyaffected. Preferably, such armatures are tubes of high melting oxides,such as aluminum oxide, beryllium oxide, thorium oxide or zirconiumoxide.

The thermocouples according to the invention, as a1- ready indicatedabove, essentially comprise an iridium oriridium-rhenium alloy (rich iniridium) element joined (paired) with a rhenium-iridium alloy (rich inrhenium). The rhenium-iridium alloy paired with a pure iridium elementpreferably contains up to 40% of iridium and alloys of 60%-.80% of`rhenium and 40%-20% of iridium have provedparticularly suitable, aswithin this range the thermoelectric E. M. F. of the thermocouple israther insensitive to small changes in the composition of the alloy. Itwas also found that when rhenium-iridium alloys containing 70 to 90% ofrhenium were paired with pure iridium the thermoelectric E. M. F. ofthethermo- .couples was to a great extent independent of the temperatureof the cold junction of the thermocouple so that in use it is notnecessary to maintain au exact control over the temperature of the coldjunction of such thermocouples. In the iridium-rheniumalloy/rhenium-iridium alloy thermocouples according to the invention,the iridium-rhenium alloy contains` up to 10% of rhenium, and preferablyabout 5 to 10% of rhenium, and the rhenium-iridium alloy contains 5090%of rhenium, and preferably 60-80% of rhenium. In the latter range it wasagain found that the thermoelectric E. M. F. of theiridium-rhenium/rhenium-iridium thermocouples is rather insensitive tosmall changes in the rhenium-iridium alloy composition.

The following Tables I and II tabulate the thermoelectric E. M. F. ofvarious Ir/Relr and IrRe/Relr thermocouples at temperatures from 1500 C.to 2100 C. in m. volts, as well as change in thermoelectric E. M. F. for1 C. change in temperature at 2000 C. in p. volts.

Table I Re 90%, I1' 10% 5. 36 7. 87 10. 37 13.32 14.8 Re Ir 20%-- 6. 458.63 11.26 14. 37 15.6 Re 70%, Ir 30". 6. 92 9. 25 11. 99 15.22 16. 2 Re60%, Ir 40%.- 7. 46 9. 66 12. 40 15. 70 16. 5 Re 40%, Ir 60%.. 5. 816.83 8.37 10. 24 9. 4

Table II IrRe (Ir 95% Re 5%)lReIr THERMOGOUPLES E in m. volts at- DE/dtComposition of Relr Alloy in ,t

1,500 1,700 1,900 2,100 Volts C. C. C

Re Ir 10% 3.36 5. 37 7.07 8.07 5. 0 Re 70%, It 30?". 4. 92 6.75 8. 6910.22 7. 7 Re 60%, Ir 40% 6. 46 7.16 9.10 10.70 8. 0 Re 50%, Ir 509'.5.17 6, 42 7. T2 8.72 5.0

As indicated above, it is preferable to protect the thermocouplesaccording to the invention against inuence of the atmosphere byproviding a non-metallic gas tight casing for the thermocouplesthemselves. It is known that high melting oxides, such as aluminumoxide, zirconium oxide, thorium oxide and others, when suitably pure,can besintered to produce practically gas tight shaped bodies. Suchshaped bodies have already been employed as protective tubes forthermoelements. However, for the purpose of the invention, it isimportant that every inuenee of the atmosphere, especially of oxygencontaining gases, be excluded with certainty. This, for example, can beaccomplished by iilling the casings containing the thermocouplesaccording to the invention with inert gases or to continuously flush thecasings with gases which have no deleterious eifect upon the materialsof the thermocouples at the temperatures to be measured. When the casingis sealed absolutely gas tight, it is generally not necessary to employa protective gas. However, in accordance with an advantageousmodification according to the invention, it has been found desirable tointroduce a getter metal into the casing to remove any last traces ofair or oxygen. The getters can be introduced into the casing either inpowder form or as shaped bodies. Getter metals, such as titanium,tantaluln, thorium or zirconium, are capable of binding gases such asoxygen, nitrogen 'or vaporsof metalloids with the formation of solidcompounds at elevated temperatures, such as v such as given in TableIII.

non-metallic protective casings for the thermocouples can be providedwith a glaze-like coating of mixtures of higher melting oxides `andappropriate -quantities otflux-V ,ing oxides in order to ensurea gastight seal. Such -oxide mixtures are especially desirable for bondingandsealing the joints in the casing when it` is not .made of one piece.Preferably, the composition of the glazes is selected so thatV itsymelting or softeningpoint can be selected `with regard to thetemperatures to be measured betweenvlOOO" C. and l700 C.l These glazeswet the non-,metallic material of the'casings upon-fusion and form a`uniform tirmly adhering coating.` The bond of such it possible to usesuch oxide mixtures to seal the casing and especially the joints thereoffor thermocouples to be used in measuring temperatures of up to 21007C., even though such temperatures substantially exceed the actualmelting point of such oxide mixtures, without danger of failure of theseal. Oxide mixtures which canbe used either to form a glazed `coatingon the casing or to bond the joints between separate parts of thecasings are given by way of example in the following table:

v Table III aluminum oxide and sealing masses 27 and 28 of an oxidemixture such asy given' inV Table III. A tube 29 of a getter metal, suchas titanium, is provided within protective tube 21. Anouterfglazecoating 30 is also provided on tube 21. Y

`We claim: Y f g l. A thermocouple comprising an iridium4 rich fele'ment selected from the group consisting of aV pure iridium element andiridium rich iridium-rhenium alloy'` elements l containing at least 90%of iridium joined with a rheniumiridium alloy element containing up to60% of iridium.

2. A thermoelectric element comprising a thermocougroup consisting of apure iridium element and iridium l5 rich iridium-rhenium alloy` elementscontaining atleast 90% of iridium joinedV with a rhenium.-iridium alloyelement containing'up to 60% of iridiun'rsealed` in gas tight casing ofnon-metallic thermostable'imaterial.

3. A thermoelectric elementY comprisingga thermocouple comprisinganiridium rich element selected from'the group consisting of a pureiridium element and iridium.

rich iridium-rhenium alloy elements containing; atleast 90% of iridiumjoined with a rhenium-iridiumalloy ele.-

rnent containing up to 60% of iridium sealed'V with a getter in a gastight casing of non-metallick thermosta'ble material. i

4. A thermoelectric element lcomprising a thermocou:

ple comprising an iridium rich element selected from the groupconsisting of a pure iridium element and iridium,

rich iridium-rhenium alloy elements containing atleast 90% of iridiumjoined with a rhenium-,iridium alloyele-V Melting Point A1203 B20; BeOCaO FezOs MgO The accompanying drawings illustrate several modicationsof thermocouple elements according to theinvention.

In such drawings:

Fig. l shows a longitudinal section of one form of such an element; and

. Fig. 2 shows a longitudinal section of another form of such anelement. j

In the modification shown in Fig. 1, the thermocouple 13 comprises aniridium wire or an iridium-rhenium alloy wire joined with arhenium-iridium alloy wire at terminal or point 14 to form the hotjunction` of the thermocouple. A protective tube 11 of a high meltingoxide, for ex-r ample, sintered aluminum oxide, surrounds the hotjunction and the wires of the thermocouple are supported in the twocapillaries provided in support 12 which `preferably is formed of thesame material as tube 11. Sealing mass 15 seals the joint between tube11 and support 12 and sealing mass 16 provides a seal at the end ofsupport 12 where the wires of the thermocouple 13 leave such support.The sealing mass is a mixture of oxides An outer glaze coating 17 ofythesarne material is provided on tube 11.

In the modication shown in Fig. 2, the thermocouple 24 comprises aniridium wire joined with a rheniumiridium alloy Wire at terminal orpoint 25 to form the hot junction of the thermocouple. One of the wiresof suchV thermocouple is provided with a sintered aluminum oxideinsulating tube 22 and the other with va plurality of short insulatingtubes 23 of the same material. `A protective tube 21 of aluminumsintered oxide surrounds the hot junction of the thermocouple and theopen end thereof is sealed oi with the aid of stopper 26 of sinteredment'containing up to 60% of iridium sealed in` a gas tight casing ofthermostable sintered oxide. l

5. A thermoelectric element comprising a thermocouple comprising aniridium rich element selectedjfrornS the group consisting of a pureiridium element and iridiuml rich iridium-rhenium alloy elementscontainingat'leastv 90% of iridium joined with a rhenium-iridium alloyele-` ment containing up to 60% of iridium sealed in algas` tight casingof thermostable sintered oxide provid'edrwith a glaze having a softeningpoint betweenQlOOO C. and;` l700 C. composed of a mixture of oxides anda'ux."'

joined with a rhenium-iridium alloy element ,containing up to 40% ofiridium.

7. A thermocouple comprising a pure iridiumV eleliije Y with arhenium-iridium alloy element containingrO,to'`

% of rhenium.

References Cited in the tile of this patent Y UNITED STATES PATENTS2,012,465 Godecke v Aug. V27, 1935 i i, j FOREIGN PATENTs' plecomprising an iridium rich element selected from the 6. A thermocouplecomprising a pure iridium element

1. A THERMOCOUPLE COMPRISING AN IRIDIUM RICH ELEMENT SELECTED FROM THEGROUP CONSISTING OF A PURE IRIDIUM ELEMENT AND IRIDIUM RICHIRIDIUM-RHENIUM ALLOY ELEMENTS CONTAINING AT LEAST 90% OF IRIDIUM JOINEDWITH A IRIDIUM.